1. Field of the Invention
The present invention relates to a photoelectric encoder.
2. Description of the Related Art
A photoelectric encoder, in which the displacement of a movable member is converted into a signal, is disclosed in Japanese Patent Application Laid-open No. 2002-243503. In the case of the photoelectric encoder, illumination light beams are projected onto a movable grating or grid (reference numeral 15 shown in FIG. 1 of Japanese Patent Application Laid-open No. 2002-243503) which is movable together with the movable member and an index grating or grid (reference numeral 14 shown in FIG. 1 of Japanese Patent Application Laid-open No. 2002-243503) which is fixed. The intensities of the illumination light beams transmitted through the both gratings are detected by a light-receiving element (reference numeral 16 shown in FIG. 1 of Japanese Patent Application Laid-open No. 2002-243503) as the information to indicate the amount of discrepancy between the both gratings. The output signal from the light-receiving element is periodically changed in accordance with the movement of the movable grating. Accordingly, it is possible to detect the minute displacement (moving) amount of the movable member from the intensity of the signal.
In general, the following artifice is applied to the photoelectric encoder of this type. That is, the intensities of the respective illumination light beams individually transmitted through the respective areas on the movable grating and the index grating are individually detected by a plurality of light-receiving surfaces of the light-receiving element. Further, the arrangements of the grating patterns of the movable grating and the index grating are designed so that the discrepancy amount between the gratings is deviated little by little between the areas. In this case, the output signals from the respective light-receiving surfaces are periodically changed respectively at mutually different phases in accordance with the movement of the movable member. Therefore, it is possible to detect the amount of movement and the direction of movement (i.e., the displacement) of the movable member according to the intensities of the signals.